1. Field of the Invention
The present invention relates to a toner carrier and an image-forming apparatus. More particularly, the present invention relates to a toner carrier which is used for image-forming apparatus, such as copying machines, facsimiles, and printers, in such a way that a toner is supplied to an image-forming medium, such as photo-sensitive material and paper having an electrostatic latent image, thereby forming a visible image on the surface of the image forming medium. The toner carrier forms uniform, high-quality images and has good durability with a minimum change in its characteristic properties after prolonged use. The present invention relates also to an image-forming apparatus which employs the toner carrier.
2. Description of the Related Art
In conventional image-forming apparatus of electrophotography system, such as copying machines and printers, the image-forming medium, such as photosensitive material, having an electrostatic latent image is supplied with a one-component toner and the toner is caused to adhere to the latent image so that the latent image is made visible (or an image is formed). This process is known as the pressure development. (See U.S. Pat. Nos. 3,152,012 and 3,731,146.)
In the process for pressure development, a toner carrier carrying a toner is brought into contact with the image-forming medium (photosensitive material) having an electrostatic latent image such that the toner adheres to the latent image on the image-forming medium to form a visible image. Consequently, the toner carrier has to be formed from a conductive elastic material having both electrical conductivity and elasticity.
As shown in FIG. 2, the process for pressure development employs a toner carrier (or development roller) 1 arranged between a toner supply roller 5 and an image-forming roller (photosensitive body) 6 which carries an electrostatic latent image. As the toner carrier 1, the image-forming body 6, and the toner supply roller 5 rotate respectively in the direction of arrows shown in the figure, the toner 7 is supplied to the surface of the toner carrier 1 by the toner coating roller 5. The thus supplied toner is made into a uniform thin film by the spreading blade 8. As the toner carrier 1 carrying a thin toner film thereon rotates in contact with the image-forming body 6, the toner (in the form of thin film) transfers from the toner carrier 1 to the latent image on the image-forming body 6, with the result that the latent image is made visible. The visible image is finally transferred to the recording medium (such as paper) by the transfer unit 9. After transfer, the residual toner remaining on the surface of the image-forming body 6 is removed by the cleaning blade 11 of the cleaning unit 10.
The image-forming apparatus for pressure development requires that the toner carrier 1 should rotate in close contact with the image-forming body 6. Therefore, the toner carrier 1 is constructed as shown in FIG. 1. The shaft 2 is made of metal having good electrical conductivity. On the shaft is formed the electrically conductive elastic layer 3 which is composed of elastic rubber or foam (such as silicone rubber, acrylonitrile butadiene rubber, ethylene-propylene rubber, and polyurethane rubber) and an electrically-conductive material. The electrically conductive elastic layer 3 is covered with the coating layer 4 of resin or the like which permits the toner 7 to be charged adequately and to become sticky adequately and also controls friction with the image-forming body 6 and the spreading blade 8. The coating layer 4 also prevents the image-forming body from being stained by the elastic body.
There is another image-forming method. According to this method, the toner carried by the toner carrier is caused to fly directly to the image-forming body (such as paper and transparency sheet) through a perforated control electrode. There is further another image-forming method. According to this method, a toner carrier in sleeve form is arranged in close proximity (but not in contact) of the image-forming body (or photosensitive body) and a non-magnetic toner is spread in thin film over the surface of the toner carrier. The toner flies to the image-forming body to form an image. (See Japanese Patent Laid-open No. 116559/1983.)
The above-mentioned toner carrier usually has its electrically conductive elastic layer covered with a coating layer of resin or the like which controls the chargeability and stickiness of the toner and reduces friction with the image-forming body, spreading blade, and control electrode. This coating layer is formed from melamine resin, phenolic resin, alkyd resin, fluorocarbon resin, polyamide resin, or the like.
The recent technical advance, including faster printers, finer images, and color images, needs to meet more stringent requirements than before. When the conventional toner carrier is used (in a printer, for example) for a long period of time, the following problems arise. (1) Fogging occurs in a white image and density becomes insufficient in a black solid image. (2) Image noise occurs cyclically as the toner carrier rotates. (3) Streaky image (defect) occurs at the part corresponding to the edge of the toner carrier.
The present invention has been completed in view of the foregoing. It is an object of the present invention to provide a toner carrier which exhibits good durability for a long period of time while continuously producing high-quality images. It is another object of the present invention to provide an image forming apparatus equipped with the toner carrier.
In order to achieve the above-mentioned object, the present inventors carried out extensive studies to find the following. The problem (1) mentioned above arises when a large amount of toner forms a film on the surface of the toner carrier after prolonged operation, with the ratio of insufficiently charged toner increasing. The problem (2) mentioned above arises when cracking or peeling occurs in the coating layer on the surface of the toner carrier after prolonged operation. The problem (3) mentioned above arises when the edge of the toner carrier wears due to friction with the toner which accumulates and remains on the edge of the toner carrier without being used for development.
It was found that the problems (1) and (2) can be solved if the toner carrier is provided with an adequate coating layer. It was also found that the problem (3) can be solved if the toner carrier is provided with an adequate conductive elastic layer. These findings led to the present invention.
The first aspect of the present invention resides in a toner carrier composed of an electrically conductive shaft and an electrically conductive elastic layer formed around said shaft, said toner carrier carrying a toner in the form of thin film on its surface, coming close to or coming into contact with the image-forming body so as to supply the toner to the surface of the image-forming body, thereby forming a visible image on the surface of the image-forming body, characterized in that said electrically conductive elastic layer has a coating layer thereon which is made of a material containing a resin with a glass transition point lower than 10xc2x0 C. and has a dynamic elastic modulus (Exe2x80x2) of 107-109.8 dyn/cm2 and a loss tangent (tan xcex4) smaller than 0.7.
The second aspect of the present invention resides in an image-forming apparatus equipped with a toner carrier which carries a toner in the form of thin film on its surface, comes close to or comes into contact with the image-forming body so as to supply the toner to the surface of the image-forming body, thereby forming a visible image on the surface of the image-forming body, wherein the toner carrier is one which is defined as above.
The third aspect of the present invention resides in a toner carrier composed of an electrically conductive shaft and an electrically conductive elastic layer formed around the shaft, the toner carrier carrying a toner in the form of thin film on its surface, coming close to or coming into contact with the image-forming body so as to supply the toner to the surface of the image-forming body, thereby forming a visible image on the surface of the image-forming body, characterized in that the electrically conductive elastic layer is made of an elastic material having a tear strength greater than 10 kg/cm (according to JIS K6252).
The fourth aspect of the present invention resides in an image-forming apparatus equipped with the toner carrier as defined above (in the third aspect of the invention).